Coupling Constants in Proteins
Hydrogen Bond Experiments
CALIS-1 and CALIS-2 are pulse sequences for the accurate calibration of the RF field strength for an indirectly observed spin are introduced. CALIS-2 is intended for calibration of e.g., 13C or 15N pulses on natural abundance samples whilst CALIS-1 is recommended primarily for enriched samples. Both experiments can be performed without prior knowledge or guess of the RF field strength and no delays in the pulse sequences are critically dependent on coupling constants.
For = /2 the signal goes through a zero crossing. The same phasing is applied to all of the spectra recorded with the different vales of . The delay = 2(1JIS)-1 and the delay '= -5 . The delay = 0-5 /2
The gradients are set so as to fulfil either of the equations:
1) IG1 - (I + S)G2 - (I - S)G3 = 0
2) IG1 - (I - S)G2 - (I + S)G3 = 0
For I and S being 1H and 13C respectively the first equation is fulfilled for G1=+5, G2=-2 and G3=+10. The gradient Gp is optional and can be of any size.
The recommended phase cycle is an even number of steps out of:
Resultsa) shows an example calibration of the 13C RF field strength using the CALIS-2 sequence on mannose and shows both the and anomeric signals. b) as for (a) but using the CALIS-1 sequence. The clean zero crossing illustrates how effectively any residual signals from protons attached to 12C are suppressed.
Document: CALIS (index.shtml)
Last modified: 2006-05-15